Drilling fluids serve various functions, such as promoting borehole stability, removing drilled cuttings from the wellbore, cooling and lubricating the bit and the drillstring, as well as controlling subsurface pressure. Specially formulated fluids are used in connection with completion and workover operations to minimize damage to the formation. Workover fluids are used during remedial work in the well, such as removing tubing, replacing a pump, logging, reperforating, and cleaning out sand or other deposits.
Formation damage can result when solids and/or filtrate derived from one of these fluids invades the formation during drilling operations. Such formation damage may be minimized by treating the well in a near-balanced condition (wellbore pressure close to formation pressure). Unfortunately, in high-pressure wells, the well often must be treated in an over-balanced and/or an under-balanced condition. If the well is over-balanced, the treating fluid is designed to temporarily seal perforations to prevent entry of fluids and solids into the formation. If the well is under-balanced, the treating fluid is designed to prevent entry of solids from the formation into the wellbore.
Current under-balanced drilling technologies use air, foams and aerated fluids. These air-based technologies solve some of the problems encountered drilling in under-balanced conditions; however, air based technologies are very costly.
Low density fluids have the capacity to control over-balanced and under-balanced formations. Water-based low density fluids are of particular interest because of environmental superiority and relatively low cost. In order to be effective, low density drilling fluids must have adequate rheology and fluid loss control properties. Viscosity and fluid loss may be controlled in low density fluids by adding certain polymers to the fluid. Solid bridging agents also generally are required to prevent fluid loss.
Unfortunately, solid bridging agents may plug pore throats in the reservoir rock. It would be desirable to formulate fluids which have the necessary rheology and fluid loss control properties without the need to use solid bridging agents.